Techniques for imparting compactness to especially a multifilament yarn, in which the yarn is subjected to the action of turbulent fluid, have heretofore been well-known. Among others, Japanese examined patent publications Nos. 36-12230, 37-11752, discloses a non-bulky textured yarn, provided with an interlaced configuration made by treating a multifilament yarn composed of a plurality of straight individual filaments in a stream of air jet fluid. Further Japanese examined Patent Publication No. 48-33424 discloses a textured yarn, having crimps intermittently along its length, which can be prepared by treating a crimped yarn with an air jet. A process in which two false twisted yarns having opposite torques are alined and subjected to the action of turbulent fluid to produce a single yarn is disclosed in Japanese examined Patent Publication No. 49-1266. Japanese examined Patent Publication No. 49-26094 discloses an intermittently compacted textured yarn obtained by the air jet treatment of a false twisted yarn. An air jet treatment on a draw-false-twisting machine is disclosed in Japanese Laid-open Patent Application No. 48-82148. In Japanese Laid-open Patent Application No. 49-2951 there is disclosed a process for producing an intermittently compacted textured yarn in which a false twisted yarn is subjected to the action of turbulent fluid under an effective driving system.
However, these techniques involve various problems which have not been solved. While the processability of the yarn may be enhanced by imparting compactness or cohesiveness to the yarn by interlacing individual filaments in the yarn, the bulkiness of the yarn decreases and the appearance of a fabric made from said yarn is damaged as the processability is enhanced. If a high degree of bulkiness and good appearance of the fabric should be maintained, a satisfactory processability cannot be attained. The interlaced yarn obtainable by prior art processes has compact portions which are not uniform, in that some of the compact portions in such yarn are denser than other compact portions in the same yarn. When such a yarn is passed through a weaving or knitting process and the preparatory steps for such a process, the interlacement of filaments may be released in some compact portions of the yarn while other compact portions retain the interlacement of filaments, leading to a poor appearance of the resultant fabric. Furthermore, since various compact portions of different degrees of compactness are formed, excessive air jetting is required to achieve the required degree of compactness for the whole yarn, which tends to render the yarn as a whole excessively denser.